FIG. 3 is a partially broken side view showing a construction of conventional shadow mask type color CRT. In FIG. 3, reference numeral 1 designates an outer frame whose inside is kept highly vacuous. An electron gun 2 for emitting three electron beams is contained in the outer frame 1. Reference numeral 3 designates a shadow mask serving as a color selecting electrode, which is formed of, for example thin iron plate having multiple slits.
Reference numeral 4 designates a translucent glass panel serving as a part of the outer frame 1. A fluorescent screen 5 is formed on an inner surface of the glass panel 4. Stripes or dots of fluorescent materials emitting red, green and blue light are then applied on the inner surface in regular succession. This stripe group or dot group are provided to precisely correspond to the slits of the shadow mask electro-optically. Reference numeral 6 designates a deflecting apparatus for deflecting and scanning the electron beam. Reference numeral 8 designates a support spring which is provided between the shadow mask 3 and outer 1. Usually, four support springs are provided. Reference numeral 9 designates an IMS (internal magnetic shield) which prevents floating magnetism, for example earth magnetism, from affecting unfavorably upon the CRT.
Operation of the above-described CRT will be described.
Three electron beams emitted from the electron gun 2 are deflected by the deflecting apparatus 6 so as to scan the whole surface of the fluorescent screen 5 and then reach the shadow mask 3. The shadow mask 3 has a color selecting function by which the three electron beams each hit only the stripe or dot of the fluorescent material having the corresponding color.
It is originally set so that positions of the stripes or dots on the fluorescent screen 5 precisely correspond to positions of the slits of the shadow mask 3. However, in actual operation of the color CRT, approximately 80% of the electron beams emitted from the electron gun are shielded by the shadow mask 3 because they collide with it. Therefore, unnecessary heat energy is transferred to the shadow mask 3 and a temperature of the shadow mask 3 is increased. As a result, the shadow mask deforms because of thermal expansion. The positional relationship between the slits of the shadow mask 3 and the stripes or dots of the fluorescent screen 5, which are set so as to correspond to each other, are shifted, causing color deviation.
A method for solving those problems is disclosed in Japanese Patent Laid Open Gazette No. 55-76553, in which an electron beam reflecting film 7 formed of material having electron beam reflection coefficient larger than that of a material constituting the shadow mask 3 is produced on an electron beam irradiated surface of the shadow mask 3.
In addition, as shown in Japanese Patent Publication Gazette No. 60-14459, a method is proposed in which the electron beam reflected film 7 is formed by a spraying solution, comprising a material of heavy metal having an atomic number of more than 70, to the electron beam irradiated surface of the shadow mask 3. More specifically, bismuth oxide powder is used as the above mentioned heavy metal and water glass and an appropriate amount of pure water are added to the bismuth oxide powder. Then, the bismuth powder is pulverized, until its average grain diameter becomes less than 1 micron, by performing ball mill and it is sprayed on the shadow mask 3. Then, it is air-dried and then thrown into the manufacturing process of the color CRT.
According to the conventional method for manufacturing the color CRT, since the electron beam reflecting film comprising a heavy metal or a heavy metal oxide is formed by spraying of a mixture of powder of a heavy metal or a heavy metal oxide and water glass, gas desorption of, for example H.sub.2 O, CO and C0.sub.2 gas from the film by heating, electron irradiation and the like increases. Thus, a cathode of the CRT is considerably poisoned. As a result, an emission life characteristic of the cathode is degraded and the life of the CRT is reduced.
In addition, since the slit of the shadow mask of high resolution is very small, for example a dot radius is 130 microns and a dot pitch is 280 microns, when the spraying method is used, the above mentioned slit is blocked by secondary agglomeration of the sprayed material so that a blocking defect occurs. As a result, the manufacture of a CRT of high resolution is prevented. In addition, a voltage resistant characteristic of the tube is degraded because powder particles are peeled from the sprayed film.